Fuel injection systems of internal combustion engines have been known for many years. These fuel injection systems include common-rail injection systems in which fuel is placed at a high pressure by means of a high-pressure pump, fed into a high-pressure accumulator and passed on from there via injector feed lines to timed injectors, and injected into the cylinders of the internal combustion engine by means of these injectors.
In order to be able to comply with strict emission standards and to be able to operate the internal combustion engine at a high fuel efficiency, a fuel injection system must be capable of being able to inject a precisely metered fuel quantity in a precisely defined time period by means of a respective injector into a respectively associated cylinder of the internal combustion engine. For this purpose, a control unit is provided, said control unit outputting an instruction for starting an injection process.
It is already known to define the time of outputting of the starting instruction by taking into account delays which are caused by the mechanics of the injection valve. One of these delays is a hydraulic delay which is due to the fact that an injector requires time to move its needle into a position which permits fuel to be injected through the valve. In order to compensate such hydraulic delays which are caused by the mechanics of the injection valve, the fuel temperature, the fuel pressure prevailing in the high-pressure accumulator and the fuel quantity of the previous injection process or processors are measured using a hybrid test bench as an offline operation.
Data which corresponds to a respectively associated hydraulic delay are derived from the specified values. These data are stored in the software of the control unit and taken into account during the determination of the time of outputting of the instruction for starting the injection process during the operation of the internal combustion engine.